Induction melting furnaces are known in various designs and the frequencies which can be used extend from approximately 50 Hz into the MHz range. The design of the furnace and crucible for receiving the melt as well as the devices for removing the melt depend to a considerable extent on the thermal and electric properties of the melt material. In order to melt ceramic materials with melting points above 1000.degree. C., especially between 2000.degree. C. and 3000.degree. C. and to avoid impurities of the melt by the crucible material, the so-called skull or sintering crust technology has proven to be suitable, in which the material to be melted forms a sintering crust crucible on the cold wall of a crucible form--cf. W. Assmus, "Chemie Ingenieur Technik" 55, (1983) (9), pp. 716-707 and V. J. Alexanderov, "Current Topics in Materials Science" Vol. 1, (1978), pp. 421-280.
The technology described in the cited documents and the crucibles used therein do not permit a semi-continuous production of a high-melting material with external further processing of the melt. Moreover, this technology exhibits further disadvantages as it is time-consuming and work-intensive on account of the particular refilling of the crucible, formation of the sintering crust and cooling off. In additions, fluctuations in quality from batch to batch can not be excluded.
The high-frequency induction melting furnace of French Patent No. 1,430,192 permits the attainment of temperatures of up to 3000.degree. C. and has a sintering crust crucible. The crucible according to FIG. 2 of the French patent comprises a spout for teeming the melt. The entire contents of the crucible can be emptied herewith by tilting in as far as a sintering crust optionally formed on the surface of the crucible is pierced through. No suggestion can be gathered from this document for a semi-continuous operation by removing a part of the melt at periodic intervals and supplementing the removed amount of melt by periodically or continuously adding material to be melted or for a suitable device to this end. A total emptying of the crucible is disadvantageous because the total refilling of the crucible with the dielectric, powdery material to be melted as well as with the mental required for the initial energy absorption which is then necessary is expensive and entails a low space-time yield. In addition, the further processing of the total molten crucible contents results in problems on account of the amount of energy expended if the crucible contents are large. Finally, rather large variations in the quality of the individual crucible charges are difficult to avoid.
EP-A 0,079,266 teaches a further induction melting furnace with a sintering crust crucible. This furnace with several windings is operated with a frequency of preferably 10 kHz to 50 kHz and comprises a device for removing the melt at the bottom of the furnace. This .device comprises a discharge tube which extends through the bottom and is surrounded by an inductor which is independent of the actual furnace. The discharge tube consists of a material which absorbs the selected frequency and can be closed with a plug of the same material. This discharge device, which must not enter into any conductive connection with the bottom of the crucible, must be cooled during the actual melting process by a suitable cooling device. The technical design of such a device is therefore very expensive. Moreover, the introduction of impurities into the molten material due to contact with the material of the discharge tube can not be sufficiently excluded, especially if, for example, the melt is to be discharged periodically or even continuously. The furnace of EP-A 0,079,266 is suitable for melting those ceramic materials whose so-called coupling temperature and melting temperature are very close to one another. In contrast thereto, short circuits and associated damage to the inductor and the generator can occur in the case of materials with a coupling temperature and a melting temperature which are far apart from one another. The furnace in EP-A 0,079,266 can thus be used only in a limited fashion, e.g. for melting glass and enamel but not very high-melting materials such as e.g. titanium dioxide or zirconium silicate.
EP-B 0,119,877 teaches a high-frequency induction melting furnace whose wall simultaneously forms the inductor and the cold wall side of a sintering crust crucible. The inductor includes a single flat winding with several conduits. According to one embodiment, the furnace comprises an optionally cooled tube passing laterally through the coil which tube is intended to remove the melt. In the case of materials with a high melting point and especially in the case of materials such as zircon sand, in which a volumetric increase occurs during cooling off, e.g. by means of modification transformation, such a tube becomes clogged; however, a heating of the tube can not be effected and/or results in considerable technical expense and/or material problems.
An attempt by the Applicant of the present invention at simultaneously supplying material to be melted and removal of the melt in a furnace of the above-mentioned type (EP-B 0,119,877) proved to be impracticable in the case of high-melting materials such as zircon sand because either the level drops too rapidly and non-molten material is entrained by the melt or, in the case of too slight a flow-off, the melt solidifies in the run-out tube. An opening of the run-out tube proved to be extremely problematic. The high resistance of the solidified melt in the run-out tube aggravated or prevented an opening by means of knocking with chisel and hammer. Even drilling out the solidified material presented difficulties on account of the hardness and brittleness of the materials examined. In the case of only a partial opening of the tube, it closed up rapidly again due to crust formation. In addition, manual labor in the area of a high- to medium-frequency scattered radiation is not admissible and is dangerous even after the oscillating circuit has been shut off on account of the hot melt.
EP-A 0,248,727 describes a generic induction melting furnace which comprises only a single flat winding with one conduit as the inductor. Customary tube generators are used for the high-frequency range and semiconductor generators are used for the medium-frequency range. According to one embodiment of the furnace in EP-A 0,248,727, the furnace is provided from below with a bottom outlet tube with a plug. This device permits only a total emptying of the crucible contents with the disadvantages already mentioned.
A total or optionally partial emptying of an induction melting furnace by means of tilting the furnace by means of a tilting device and teeming the melt via a teeming lip is known from U.S. Pat. No. 2,785,214. However, the teeming is only possible in as far as no thermal radiation crust from the material to be melted has formed over the melt. However, such a crust is desirable from an energy saving standpoint.